1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a reflective LCD device having a cholesteric liquid crystal (CLC) color filter.
2. Discussion of the Related Art
Presently, LCD devices are developed as next generation display devices because of their light weight, thin profile, and low power consumption characteristics. In general, an LCD device is a non-emissive display device that displays images using a refractive index difference utilizing optical anisotropy properties of liquid crystal material that is interposed between an array (TFT) substrate and a color filter (C/F) substrate. Among the various type of LCD devices commonly used, active matrix LCD (AM-LCD) devices have been developed because of their high resolution and superiority in displaying moving images. The AM-LCD device includes a thin film transistor (TFT) per each pixel region as a switching device, a first electrode for ON/OFF, and a second electrode used for a common electrode.
FIG. 1 is a perspective view of an LCD device according to the related art.
In FIG. 1, first and second substrates 10 and 30 are arranged to face each other with a liquid crystal material layer 50 interposed therebetween. On an inner surface of the first substrate 10, a color filter (C/F) layer 12 and a common electrode 16, which functions as one electrode for applying an electric field to the liquid crystal layer 50, are subsequently formed. The C/F layer 12 includes a color filter for transmitting only light of a specific wavelength, and a black matrix (not shown) that is disposed at a boundary of the color filter and shields light of a region in which alignment of the liquid crystal material is uncontrollable. On an inner surface of the second substrate 30, a plurality of gate lines 32 and a plurality of data lines 34 are formed in a matrix array. A TFT “T”, which functions as a switching device, is disposed at a region where each gate line 32 and data line 34 crosses, and a pixel electrode 46 that is connected to the TFT “T” is disposed at a pixel region “P” defined by the region where the gate and data lines 32 and 34 cross. First and second polarizing plates 52 and 54, which transmit only light parallel to a polarizing axis, are disposed on an outer surface of the first and second substrates 10 and 30, respectively. An additional light source such as a backlight, for example, is disposed below the second polarizing plate 54.
The LCD device of FIG. 1 is a transmissive LCD device that displays images by transmitting only desired light through the first substrate using an optic/dielectric anisotropy of the liquid crystal layer after light from the backlight passes through the second substrate.
FIG. 2 is a schematic plan view of an LCD device according to the related art. FIG. 2 shows gate and data pads for connection with an external circuit.
In FIG. 2, a liquid crystal panel 60 for an LCD device includes a first substrate 10, a second substrate 30 larger than the first substrate 10, and a liquid crystal layer 50 interposed between the first and second substrates 10 and 30. The liquid crystal panel 60 can be divided into a display region “D,” and first and second non-display regions “N1” and “N2” in plan view. The first non-display region “N1” is defined by the first and second substrates 10 and 30, and the second non-display region “N2” is defined by a larger portion of the second substrate 30. Elements such as a TFT, gate and data lines, a pixel electrode, a color filter layer and a common electrode illustrated in FIG. 1 are formed in the display region “D.” Gate and data pads 62 and 64 connected to an external circuit (not shown) are formed in the second non-display region “N2” to apply a display signal to the display region “D.” Since a black matrix 66 formed in the first non-display region “N1” of the first substrate 10 absorbs incident light, a boundary of the display region “D” maintains a black state.
FIG. 3 is a schematic cross-sectional view of an LCD device according to the related art. A boundary of a display region is mainly shown in FIG. 3.
In FIG. 3, a boundary of first and second substrates 10 and 30 with a liquid crystal layer 50 therebetween is sealed with a seal pattern 68. A color filter layer 40 on an inner surface of the first substrate 10 is extended to a first non-display region “N1” so that a deterioration at a boundary of a display region “D” by a step difference between the display region “D” and the first non-display region “N1” can be prevented during a rubbing process for aligning the liquid crystal layer 50. Array elements 42 such as a TFT and a pixel electrode (of FIG. 1) are formed on an inner surface of the second substrate 30. When light is emitted into the first non-display region “N1,” a black matrix 66 of the first non-display region “N1” absorbs the light. Accordingly, a black state is maintained in the boundary of the display region “D.”
Reflective LCD devices without a backlight are being researched and developed. Transflective LCD devices use a backlight to provide light. However, only about 7% of the light that is emitted by the backlight passes through each cell of the LCD device. Since the backlight should emit light of a relatively high brightness, corresponding power consumption increases. Accordingly, a large capacity heavy battery is commonly used to supply sufficient power for the backlight. Moreover, use of the large capacity battery limits operating time. On the other hand, because power consumption of the reflective LCD devices greatly decreases due to use of ambient light as a light source, operating time increases. Such reflective LCD devices are used for portable information apparatuses such as electronic diaries and personal digital assistants (PDAs). In the reflective LCD devices, a pixel area, which is covered with a transparent electrode in conventional transmissive LCD devices, is covered with a reflective plate or reflective electrode having opaque reflection characteristics.
However, brightness of reflective LCD devices is very poor because the devices use only ambient light as a light source. The poor brightness results from operational characteristics of the reflective LCD devices in which ambient light which passes through a color filter substrate, is reflected on a reflective electrode on a second substrate, is passed through the color filter substrate again and then displays an image. Accordingly, brightness is decreased as a result of reduction of the transmittance when the ambient light passes through a color filter layer twice. Since overall thickness of the color filter layer is inversely proportional to transmittance and is directly proportional to color purity of the light, the problem of inadequate brightness of the reflective LCD devices can be remedied by forming a thin color filter layer with high transmittance and low color purity. However, there is a limit in fabricating the color filter layer below a threshold thickness due to characteristics of the resin used to form the color filter layer.
Accordingly, one possible solution to this problem is fabricating LCD devices using cholesteric liquid crystal (CLC) that has selective reflection and transparency characteristics. In reflective LCD devices using a CLC color filter (CCF) layer, the fabrication processes are simplified due to omission of the reflective layer, and high color purity and high contrast ratio are achieved. Moreover, since CLC has a spiral structure and spiral pitch determines a selective reflection bandwidth of the CLC, the reflection bandwidth can be controlled by a distribution of the spiral pitch at one pixel. To illustrate this in more detail, a wavelength range of visible light is from about 400 nm to about 700 nm. The wavelength of the red light region is centered at about 650 nm, the wavelength of the green light region is centered at about 550 nm, and the wavelength of the blue light region is centered at about 450 nm. The CCF layer is formed having characteristics that can selectively reflect or transmit right-handed or left-handed circularly polarized light at a bandwidth that corresponds to a pitch deviation by selecting bandwidths corresponding to the red, green, and blue light regions. In addition, the CCF layer is formed having characteristics that control conditions for right or left pitch deviations with respect to the center wavelength. Accordingly, the pitch of the liquid crystal can be artificially adjusted so that a CCF layer can selectively reflect light of an intrinsic wavelength of the color corresponding to each pixel.
FIG. 4 is a schematic plan view of a reflective LCD device using a CCF layer according to the related art. FIG. 4 shows gate and data pads for connection with an external circuit.
In FIG. 4, since the reflective LCD device using a CCF layer displays images by reflecting ambient light, array elements (not shown) and a CCF layer are formed on first and second substrates 70 and 72, respectively. Accordingly, the first substrate is larger than the second substrate 72. As a result, even though a display region “D” is disposed as in the LCD device of FIG. 3, a first non-display region “N1” corresponds to a boundary of the second substrate 72 and a second non-display region “N2” corresponds to a larger portion of the first substrate 70. Especially, even though a black matrix is disposed at the first non-display region adjacent to the display region in the LCD device of FIG. 3, an additional black matrix between adjacent pixels is omitted due to a selective reflection property of the CCF layer and a metal bar 78 of one material of a gate pad 74 and a data pad 76 is disposed at the first non-display region “N1” not adjacent to the second non-display region “N2” to prevent light reflection in the reflective LCD device using a CCF layer. However, an extra metal bar to prevent light reflection is not disposed at the first non-display region “N1” adjacent to the second non-display region “N2” where the gate pad 74 and the data pad 76 are formed.
FIG. 5 is a schematic cross-sectional view of a reflective LCD device using a CCF layer according to the related art. A boundary of a display region is mainly shown in FIG. 5.
In FIG. 5, a first substrate 70 including array elements 86 and a second substrate 72 including a CCF layer 82 face each other. Incident light into the CCF layer 82 displays colors by the CCF layer 82 whose pitch is adjusted according to a wavelength of each color. These colors constitute desired images by a refractive index difference at a liquid crystal layer (not shown) interposed between the first and second substrates 70 and 72. To illustrate this in more detail, the array elements 86 are formed on an inner surface of the first substrate 70 and a light absorption layer 80 is formed on an entire inner surface of the second substrate 72. The CCF layer 82 is formed at a display region “D” and a first non-display region “N1” on the light absorption layer 80. A common electrode 84 is formed on the light absorption layer 80 and the CCF layer 82. The array elements 72 on the second substrate 30 of FIG. 3 can be used as the array elements 86.
When external light enters the CCF layer 82 at the first non-display region “N1,” the CCF layer 82 reflects a circularly polarized light such as left-handed or right-handed circularly polarized lights and the circularly polarized light is emitted to an exterior without passing through an additional light shielding layer. Accordingly, brightness undesirably increases at a boundary of the display region and a display quality of the reflective LCD device decreases.